1. Field of the Invention
The present invention is related to provide a driving circuit for switching direct-current (DC) power, and more particularly, a driving circuit capable of avoiding voltage peaks for protecting circuits and stabilizing system operations.
2. Description of the Prior Art
Motors, indispensable dynamic devices in the industrial society and information age, are used for converting electrical energy into mechanical energy. Some are commonly used, such as direct-current (DC) motors, alternating-current (AC) motors, stepping motors, and so on. Usually, DC motors and AC motors are adopted in products, which need not to be controlled in a precise way, such as fans. Generally, the DC motors rotate by changing current directions and current intensity of rotor coils set on rotors, and the rotors generate magnetic forces interacting with permanent magnets set on stators. Therefore, taking control of the current directions and intensity of the rotor coils controls rotation speed and directions of the motors. However, as time goes by, the faster rotation speed and stronger intensity are demanded, resulting in reliability issues.
Please refer to FIG. 1. FIG. 1 depicts a schematic diagram of a prior art driving circuit 100 of a DC motor. The driving circuit 100 includes transistors 102, 104, 106, and 108, diodes 110, 112, 114, 116, and 130, controllers 118 and 120, resistors 122 and 124, a power generator 132, a capacitor 134, and a Hall Sensor 136. The transistors 102, 104, 106, and 108 are power transistors utilized for driving a full-bridge circuit. Each of the diodes 110, 112, 114, and 116 is coupled between a base and a drain of the transistors 102, 104, 106, and 108. The controllers 118 and 120 control the transistors 102, 104, 106, and 108 to switch on or off according to a sensing result of the Hall sensor 136. The resistors 122 and 124 are seen as pull-high resistors of the transistors 102 and 106 while a resistor 126 and an inductor 128 represent an equivalent circuit of a rotor coil of the DC motor. The diode 130 is utilized for preventing current of the inductor 128 from inversely drifting to the power generator 132, so as to protect the power generator 132. The capacitor 134 is utilized for stabilizing source voltages VM of the transistors 102 and 106, and for absorbing a reverse current.
As shown in FIG. 1, the driving circuit 100 is composed of four solid-state switches split into two paths, or bridges. The transistors 102 and 106 are upper bridge switches, while the transistors 104 and 108 are lower bridge switches. Operations of the driving circuit 100 are as follows. Firstly, the Hall sensor 136 detects a magnetic pole position, N or S, of the rotor of the DC motor. According to the magnetic pole position of the rotor, the controllers 118 and 120 output control signals to each gate of the transistors 102, 104, 106, and 108, so as to control the transistors 102, 104, 106, and 108 to switch on or off. For example, if current flowing from a node 138 to a node 140 is demanded, the controllers 118 and 120 turn off the transistors 104 and 106 and turn on the transistors 102 and 108, then current outputted from the power generator 132 passes through the transistor 102, the node 138, the node 140, and the transistor 108 to ground. On the contrary, if current flowing from the node 138 to the node 140 is demanded, the controllers 118 and 120 turn off the transistors 102 and 108 off and turn on the transistors 104 and 106, then current outputted from the power generator 132 passes through the transistor 106, the node 140, the node 138, and the transistor 104 to ground. Therefore, by controlling the transistors 102, 104, 106, and 108, the controllers 118 and 120 can control the current direction of the rotor coil, so as to control the rotation of the DC motor. However, in the driving circuit 100, switches on the same bridge, such as the transistors 102 and 106, are not allowed being turned on at the same time, or the circuit will become short and seriously break down. Therefore, the switches on the upper or lower bridges are either both off at any time or taking turns by one on and one off. In addition, since the transistors 102, 104, 106, and 108 are solid-state semiconductor devices, switching on or off is conducting or cutting off charge carriers. Note that semiconductors need a carrier recovery time to transit from a conducting state to a cut-off state as well as switch from on to completely off. Therefore, the carrier recovery time needs to be considered in a time sequence of switching operations of the upper bridge switch and the lower bridge switch. An appropriate delay time can be added to the time sequence for avoiding one transistor on the upper or lower bridge switching on while the other is not completely switching off, resulting a short circuit because both transistors on the same bridge are on at the same time. In general, the delay time is named as dead time.
Please refer to FIG. 2 to FIG. 6. FIG. 2 depicts a schematic diagram of a time sequence of corresponding signals of the driving circuit 100. FIG. 3 to FIG. 6 depict schematic diagrams of current paths of the driving circuit 100 in different operating stages. FIG. 2, from top to bottom, shows a sensing result PR_H of the Hall sensor 136, operating states PR_SW1, PR_SW2, PR_SW3, and PR_SW4 of the transistors 102, 104, 106, and 108, a current PR_L of the inductor 128, voltages PR_O1 and PR_O2 of the nodes 138 and 140, and source voltages VM of the transistors 102 and 106. For clearly explanation, operations of the driving circuit 100 can divide into five stages: PR_S1, PR_S2, PR_S3, PR_S4, and PR_S5, as shown in FIG. 2. FIG. 3 shows a current path L1 of in the stage PR_S1, FIG. 4 shows a current path L2 of in the stage PR_S2, FIG. 5 shows a current path L3 of in the stage PR_S3 and the stage PR_S4, and FIG. 6 shows a current path L4 of in the stage PR_S5. The follows explain the operations of the driving circuit 100 in each stage, where FIG. 3 to FIG. 6 only depict partial circuits of the driving circuit 100 for clarity.
Firstly, in the stage PR_S1, the controller 118 and 120 switch on the transistors 102 and 108 and switch off the transistors 104 and 106, so that the voltage PR_O1 of the node 138 is higher than the voltage PR_O2 of the node 140. The current outputted from the power generator 132 flows along the current path L1, from the node 138 to the node 140, so that the current PR_L is positive.
Next, the magnetic pole of the rotor changing with rotations of the DC motor makes the sensing result PR_H of the Hall sensor 136 changed. The stage goes into the stage PR_S2. In the stage PR_S2, the controllers 118 and 120 switch off the transistors 102, 104, and 106 and switch on the transistor 108, so that current of the transistor 108 flows along the current path L2, from the diode 112 to the transistor 108 and ground. Therefore, before the DC motor changes the state, parts of the current PR_L drift to ground through the transistor 108, so as to prevent too much reverse current from drifting into the sources of the transistors 102 and 106.
After undergoing the stage PR_S2, the operation of the driving circuit 100 is forward to the stage PR_S3. At this moment, the controllers 118 and 120 switch all the transistors 102, 104, 106, and 108 off. However, a residual current, flowing along the current path L3, from the diode 112 to the diode 114, increases the voltage VM due to the residual current drifting to the capacitor 134. After that, with stepping forward to the stage PR_S4, the controllers 118 and 120 switch on the transistors 104 and 106 and switch off the transistors 102 and 108, so that the voltage VM continues increasing. The stage PR_S3 mainly functions to stagger the time when the upper bridge and the lower bridge turn on in order to avoid a shoot-through condition.
In the last stage PR_S5, the transistors 104 and 106 are on while the transistors 102 and 108 are off. Therefore, the voltage PR_O1 of the node 138 is low while the voltage PR_O2 of the node 140 is high, and current outputted from the power generator 132 flows along the current path L4 from the node 140 to the node 138, so that the current PR_L is negative, accomplishing the state transition.
In accordance with FIG. 2 to FIG. 5, in the stages PR_S3 and PR_S4, the residual motor current drifts from the diode 112 to the diode 114 and further drifts to the capacitor 134, causing the source voltages VM of the transistors 102 and 106 increasing. Furthermore, when the DC motor rotates in a higher speed, current in the DC motor is larger, so that more reverse currents drift to the capacitor 134, which increases the voltage VM and may break down the full-bridge circuit. The phenomenon decreases the reliability of the device operation. In other words, the prior art driving circuit may cause devices broken down due to the reverse current of the DC motor, further taking away operational functions of the DC motor.